Fiber optic communication systems are becoming increasingly popular for data transmission due to their high speed and high data capacity capabilities. Wavelength division multiplexing is used in such fiber optic communication systems to transfer a relatively large amount of data at a high speed. In wavelength division multiplexing, multiple information-carrying signals, each signal comprising light of a specific restricted wavelength range, may be transmitted along the same optical fiber. Strictly speaking, a multiplexer is an apparatus that combines separate channels into a single wavelength division multiplexed composite optical signal and a de-multiplexer is an apparatus that separates a composite optical signal into its component channels. However, many multiplexers and de-multiplexers ordinarily operate in either sense, simply by reversing the direction of light propagation.
In this document, the individual information-carrying lights are referred to as either “signals” or “channels.” The totality of multiple combined signals in a wavelength-division multiplexed optical fiber, optical line or optical system, wherein each signal is of a different wavelength range, is herein referred to as a “composite optical signal.” Although each information-carrying channel actually comprises light of a certain respective range of physical wavelengths, for simplicity, a single channel is referred to as a single wavelength, λ, and a plurality of n such channels are referred to as “n wavelengths” denoted λ1–λn.
To distinguish amongst the various optical ports, the optical port from which a de-multiplexer receives channels or to which a multiplexer outputs channels is referred to in this document as a “common” port. Also, the optical ports to which a de-multiplexer outputs channels or from which a multiplexer receives channels are referred to as “non-common” ports.
A crucial feature of fiber optic networks is the separation of the composite optical signal into its component wavelengths or channels, typically by a wavelength division de-multiplexer. This separation must occur to allow for the exchange of signals between loops within optical communications networks. The exchange typically occurs at connector points, or points where two or more loops intersect for the purpose of exchanging wavelengths.
Conventional methods utilized by wavelength division de-multiplexers in separating a composite optical signal into its component channels include the use of filters and fiber gratings as separators. A “separator” or “channel separator”, as used in this specification, is an integrated collection of optical components functioning as a unit which separates one or more channels of a composite optical signal from one another. Filters allow a target channel to pass through while redirecting all other channels. Fiber gratings target a channel to be reflected while all other channels pass through. Both filters and fiber gratings are well known in the art and will not be discussed in further detail here.
A problem with the conventional separators is the precision required of a transmitter device—that is, a light emitting device which generates an optical signal at a particular wavelength—because of the narrow pass bands of such separators. This high precision is difficult to accomplish. To overcome this shortcoming, related-art channel separators utilizing a polarization beam splitter and a paired reflection interferometers have been disclosed. These related-art reflection interferometers and channel separators, which have a greater ease of alignment and tolerance for wavelength offset than conventional separators, are disclosed in the following U.S. Pat. Nos. 6,130,971; 6,169,604; 6,169,828; 6,263,129; and 6,310,690. Channel separators utilizing polarization beam splitters, reflection interferometers and Faraday rotators are disclosed in a co-pending U.S. patent application titled “Multi-Functional Optical Device Utilizing Multiple Polarization Beam Splitters and Non-Linear Interferometers” Ser. No. 09/630,891, filed Aug. 2, 2000 and in U.S. Pat. No. 6,396,629. A related-art optical comb filter that utilizes a reflection interferometer is disclosed in U.S. Pat. No. 6,205,270. All of the aforementioned patents and patent applications are assigned to the assignee of the present invention and incorporated herein in their entirety by reference.
Through use of a reflection interferometer, the above-referenced related-art separators have advantages over conventional separators in terms of increased widths of the pass bands and isolation bands and greater ease of alignment of transmitters to the pass bands. However, some of these related-art channel separators utilize two independent reflection interferometers, which must be matched to one another, both position-wise and with respect to their optical characteristics. Others of these related-art channel separators utilize Faraday rotators and unpaired reflection interferometers. The relaxation of the requirement for paired interferometers within these other channel separators simplifies mechanical alignment but the incorporation of Faraday rotators causes an optical isolation effect, which prevents a single such apparatus from being used both as a multiplexer and a de-multiplexer. Therefore, there exists a need for a separation mechanism that possesses the above-mentioned benefits of the reflection interferometer, but does not require paired reflection interferometers or Faraday rotators. The present invention addresses such a need.